1. Field of the Invention
The present invention relates to disk drive. More particularly, the present invention relates to a disk drive comprising an integrator for controlling a VCM actuator and a transformer for controlling a piezoelectric actuator.
2. Description of the Prior Art
A dual stage actuator comprising a primary actuator, such as a voice coil motor (VCM), and a secondary micro-actuator, such as a piezoelectric (PZT) actuator enables significantly higher tracks-per-inch (tpi) in disk drives. A PZT actuator is a capacitive element that morphs (e.g., bends) proportional to the magnitude of an applied voltage. The morphing action of the PZT element provides a corresponding radial displacement of the head.
A DC-DC converter has been suggested to drive the PZT actuator so that while settling into the tracking mode at the end of seeks the PZT actuator can be decoupled from the primary actuator (the VCM). The driving signal (a voltage) for the PZT actuator is generated separate from the driving signal (a current) for the VCM actuator. The primary actuator is driven with a medium bandwidth loop while the PZT actuator is driven with a high bandwidth loop in response to the PES. In this manner, the PZT actuator is able to compensate for the vibrations inherent in the mechanical components of the primary actuator, as well as the non-linear, hysteretic behavior exhibited by both the pivot bearing and the flex cable. The DC-DC converter facilitates this configuration by generating PZT control voltages over a wide band of frequencies, including low frequencies. However, the DC-DC converter employs switching circuitry which can induce high frequency noise in the read signal. Further, the DC-DC converter generates large DC voltages which must be properly shielded to prevent harmful shocks during installation of the disk drive. The shielding must be approved by UL/CSA/VDE before manufacturing the disk drive, which can add significantly to design cycles and time to market.
There is, therefore, a need for an improved dual stage actuator system for use in a disk drive which avoids the high frequency switching noise and high DC voltages associated with the prior art DC-DC converters for driving PZT actuators.
The present invention may be regarded as a disk drive comprising a disk, a head, and an actuator for actuating the head radially over the disk, the actuator comprising a voice coil motor (VCM) actuator and a piezoelectric (PZT) actuator. A position error generator generates a position error signal (PES) representing a difference between an actual position of the head and a desired position of the head. The disk drive further comprises a PZT controller comprising a voltage generator for generating a first voltage in response to the PES, and a transformer comprising a primary winding and a secondary winding, the transformer for stepping up the first voltage to generate a second voltage greater than the first voltage, the second voltage applied to the PZT actuator. The disk drive further comprises a VCM controller comprising an integrator for integrating the PES to generate a VCM control signal applied to the VCM actuator in order to force the average of the second voltage toward zero thereby minimizing the volt-second product of the transformer.
In one embodiment, the voltage generator comprises a differentiator for differentiating the PES to generate an input current, a current sensor connected to the primary winding of the transformer for sensing a current in the primary winding, and an amplifier. The amplifier comprises a first input responsive the current sensed in the primary winding, a second input responsive to the input current, and an output for outputting the first voltage. The amplifier drives the current in the primary winding toward the input current to implement a charge driver. In one embodiment, the differentiator comprises a capacitor, and the input current comprises a current flowing through the capacitor.
The present invention may also be regarded as a method for use in a disk drive for actuating a head radially over a disk using a piezoelectric (PZT) actuator and a voice coil motor (VCM) actuator. A position error signal (PES) is generated representing a difference between an actual position of the head and a desired position of the head, and a first voltage is generated in response to the PES. A transformer comprising a primary winding and a secondary winding is used to step-up the first voltage to generate a second voltage greater than the first voltage. The second voltage is applied to the PZT actuator. The PES is integrated to generate a VCM control signal applied to the VCM actuator in order to force the average of the second voltage toward zero thereby minimizing the volt-second product of the transformer.